The electrical discharge machining art has advanced from the early stages in which relaxation oscillators were used to provide machining power pulses to the gap for eroding portions of an electrically conductive workpiece in the pattern of a tool electrode. Independently timed and controlled pulse generators are now almost universally used and in these generators the electronic switches employed are now generally in the form of solid state switches, particularly transistors. During the electrical discharge machining process, it is necessary as the workpiece material is removed that a predetermined gap be maintained between the tool electrodes and the workpiece through an automatic servo feed system which provides a continuous advance into and toward the workpiece as material removal progresses. During the electrical discharge machining process, a fluid coolant, generally a liquid, is circulated through the machining gap to flush the workpiece particles from the gap. The coolant is usually furnished under pressure by a pump to one or more holes provided in the electrode. One necessary and defining characteristic of electrical discharge machining is that the coolant is a dielectric fluid such as kerosene, transformer oil, distilled water, or the like. The dielectric fluid is broken down in minute, localized areas by the action of the machining power pulses passed between the closely opposed surfaces of the tool electrode and workpiece.
For control of the servo feed system, there is generally utilized an electrical signal from the machining gap in order to control the rate and the direction of servo feed. In most cases, the gap signal is compared to an adjustable reference voltage so that the operator can select the rate of servo feed desired for the particular operating condition. It will thus be seen that with respect to the servo feed of the gap elements in electrical discharge machining, a parameter of the gap, whether the average gap voltage as shown and described in Williams U.S. Pat No. 2,84l,686, issued July 1, 1958, or a peak gap voltage as shown and described in Webb U.S. Pat No. RE. 25,542, issued Mar. 24, 1964, must be sensed and applied as a control signal for an electrical or electro-hydraulic drive motor. In the present invention, the inventive circuit is incorporated in a servo control system of this electro-hydraulic type.
By way of summary, it will be seen that the present inventive circuit receives a voltage signal from the machining gap itself which signal represents a changing condition and actuates a servo feed circuit to initiate a servo feed backup operation to protect the gap elements from possible damage when gap voltage drops to a predetermined level and initiates a servo feed downfeed operation when gap voltage rises to a predetermined level.
The circuit according to the present invention likewise includes a novel arrangement of an AC dither circuit which is so located in series with the servo valve coil that AC pulses are passed to the servo valve directly with a constant presettable amplitude without being affected by the magnitude of the signal from the gap which, of course, is constantly changing in magnitude as machining progresses.
It will be understood in the specification that when I refer to "electronic switch" I mean any electronic control device having two or more electrodes comprising at least two principal or power conducting electrodes acting to control current flow in the power circuit, the conductivity between the principal electrodes being controlled by a control electrode within the switch whereby the conductivity of the power circuit is controlled statically or electrically without movement of mechanical elements within the switch. Included within this definition are transistors in which turn-on is accomplished by a control voltage applied to the transistor control electrode and in which turn-off is accomplished automatically in response to removal of that control voltage. Also included in the definition are devices of the gate type in which turn-on is accomplished by a control voltage applied to the control electrode, which control voltage may be then removed and in which turn-off is accomplished by the application of a subsequent control voltage to the control electrode. An additional class of electrical switches called electronic trigger devices falls within this definition and includes ignitrons, thyratrons, semiconductor control rectifiers, and the like. By "electronic trigger device", I mean any electronic switch of the type which is triggered on at its control electrode by a pulse and is turned off by a reverse voltage applied for sufficient time across its principal electrodes.